I recently solved my poor acceleration problem after finding out the fuel pump was bad. After pulling the tank and cleaning it, I could finally start to break in my new engine of only 160 miles on it. After about 10 minutes of driving, I get the check engine light. It comes on at about 2200 rpm's and I am only taking it up to 50 mph for the break in period. I pull the code and get 5, which is bad O2 sensor, or ECU, lean or rich condition. I have 2 ECU's and I bought a brand new Denso replacement from NAPA. Since a lot of the build ups in the motor are from LC, a tech there suggested that I go to a heated O2 sensor from an 88' to 95' truck. That way the computer will get a clean reading and the exhaust won't have to be the factor to activate the sensor. I have tried both ECU's with both O2 sensors and I still get the light. When I let off the gas, the light goes off but will return 10 minutes later. Any thoughts on my next move?